A lithium battery may be used as a power source for portable electronic devices such as mobile phones, personal digital assistants (PDAs), laptop computers, and other general electronic devices. Rechargeable lithium secondary batteries have several advantages over other types of batteries. For example, rechargeable lithium secondary batteries have three times the energy density per unit weight as Pb storage batteries, Ni—Cd batteries, Ni—H batteries, and Ni—Zn batteries. In addition, rechargeable lithium secondary batteries may be charged in a short time.
Generally, lithium-ion batteries comprise a positive electrode, a negative electrode, and an electrolyte. Examples of positive active materials include oxides of transition metal compounds and lithium, such as LiNiO2, LiCoO2, LiMn2O4, LiFePO4, LiNixCo1-xO2 (x=1 or 2 ), Ni1-x-yCoxMnyO2 (0≦x≦0.5 and 0≦y≦0.5). Examples of negative active materials include lithium metal, lithium metal alloy, carbon materials, and graphite materials.
Electrolytes may be divided into liquid electrolytes and solid electrolytes. There are several disadvantages to using liquid electrolytes in batteries. In particular, the outflow of the liquid electrolyte from the battery may result in a fire and the evaporation of the liquid electrolyte from the battery may result in the actual destruction of the battery. In order to overcome these disadvantages, solid electrolytes may be used. In general, solid electrolytes offer several advantages over liquid electrolytes because they are unlikely to leak and are more easily processed. Solid polymer electrolytes may be categorized into full solid types and gel types. Specifically, a full solid-type electrolyte does not contain an organic electrolytic solution whereas a gel-type electrolyte contains an organic electrolytic solution.
In general, a conventional aqueous electrolytic solution is not suitable for a lithium battery because a negative lithium electrode reacts vigorously with an aqueous electrolytic solution at high working voltages. Therefore, in lithium batteries, it is desirable to use an organic electrolytic solution that includes a lithium salt and an organic solvent. In particular, it is preferable to use an organic solvent having the properties of a high ionic conductivity, a high dielectric constant and a low viscosity. It is, however, very difficult to obtain a single organic solvent having all of these properties. As a result, either a mixed solvent composed of an organic solvent having a high dielectric constant and an organic solvent having a high dielectric constant; or a mixed solvent composed of an organic solvent having a high dielectric constant and an organic solvent having low viscosity is used in lithium batteries.
In order to produce an organic solvent with high ionic conductivity, U.S. Pat. Nos. 6,114,070 and 6,048,637 disclose mixed solvents comprising a linear carbonate and a cyclic carbonate, e.g., dimethyl carbonate or diethyl carbonate and ethylene carbonate or propylene carbonate. In general, mixed solvents may be used in batteries that operate at temperatures of 120° C. or less. If the battery reaches a temperature higher than 120° C., however, the battery may swell due to vapour pressure generated by gas. As a consequence, the swelled battery results in the failure of the battery operation.
Furthermore, U.S. Pat. Nos. 5,352,548, 5,712,059, and 5,714,281 disclose electrolytes having vinylene carbonate as a main organic solvent at a concentration of 20% or more. In these cases, however, the charge/discharge characteristics and the high-rate characteristics of the batteries are significantly reduced because vinylene carbonate has a lower dielectric constant than ethylene carbonate, propylene carbonate, or γ-butyrolactone. Alternatively, U.S. Pat. No. 5,626,981 discloses the use of vinylene carbonate as an additive in an electrolyte in order to form a surface electrolyte interface (SEI) on the surface of positive electrodes during an initial charge/discharge cycle. A vinyl-based compound such as vinyl acetate is used as an additive in Japanese Patent Laid-open Publication No. 2001-223154.
The use of an electrolyte comprising an electrochemical anionic polymerizable monomer to form a polymer film on the surface of a carbonaceous negative material during initial charging is disclosed by U.S. Pat. No. 6,291,107. In order to prevent the decomposition of the electrolytic solution, an electrochemical anionic polymerizable monomer may be used in the electrolytic solution, which facilitates the formation of a polymer film on the surface of a carbonaceous negative material. As a result, the thickness of the battery may be maintained within an acceptable range, regardless of the number of charge/discharge cycles of the battery. Consequently, reliability, charge/discharge efficiency and the lifetime of the battery may be improved. The polymer film, which is formed by anionic polymerization, however, causes an increase in the resistance of electrodes. Therefore, if a battery is charged at a high rate, its capacity and low temperature characteristics may deteriorate.
Therefore, in order to overcome the aforementioned problems, the present invention is directed to blending a specific additive in the electrolyte solution. Accordingly, the present invention provides for improved charge-discharge efficiency during a repeated charge-discharge process while providing a high voltage and a high energy density.